In general, multiple electric motors are mounted in an electric drive vehicle such as a hybrid electric vehicle (HEV) and an electric vehicle (EV), and particularly, a high-power electric motor is used as a driving force. As a power source that supplies power to the electric motor used as the driving force, a battery that is formed of a battery pack including plural secondary battery cells such as nickel hydrate battery cells or lithium battery cells is used. A State of Charge (SOC) is used as a parameter that indicates a charging state of the battery. For estimating the SOC during traveling of a vehicle, in general, a method is widely used in which open circuit voltage OCV is calculated from closed circuit voltage CCV during traveling of the vehicle, polarization voltage, internal resistance, and a battery current integrated value and in which the SOC is estimated from the thus-calculated OCV.
The CCV of the each secondary battery that constitutes the battery pack is measured for measurement of the CCV during traveling of the vehicle, the OCV and the SOC of the each secondary battery are then calculated from the measured values, and the SOC of the battery as a whole is further calculated. However, because charging and discharging are frequently repeated during traveling of the vehicle, it is difficult to detect the CCVs of all the secondary battery cells in an identical condition, and an error occurs to a certain extent in detection of the CCV of the each secondary battery cell. The error in the detection of battery voltage values of these secondary battery cells is accumulated, an error is also produced in the calculated SOC of the battery with respect to the actual SOC, and this error is gradually accumulated. Considering the above, a method of accurately calculating the SOC by measuring the CCV in a state where the battery is driven with a constant current has been suggested (see PTL 1).
However, a power source system that is described in PTL 1 and used to supply power to the electric motor includes plural batteries and plural converters, controls the plural converters to charge or discharge some of the plural batteries with the constant current, and charges and discharges the rest of the batteries in response to a power request by a driving force generating section, and in the meantime, a battery controller estimates the SOC of the battery on the basis of the voltage of the battery during charging or discharging with the constant current.